The present invention relates to sliding electrical contact devices comprising an electrically conducting track and a brush bearing on the track, wherein in operation the track and the brush have a relative movement, transverse to the bearing direction of the brush on the track.
Sliding electrical contacts are widely used in electric machines, particularly in heteropolar or homopolar rotary machines. Contact devices comprising brushes made from metal wires were abandoned a long time ago and substituted with devices having a ring and graphite brushes. The graphite brushes however are subject to disadvantages and troublesome limitations, in particular in respect of the maximum density of current which they can transmit. Their contact potential difference is high. Frictional forces are high and wear is fast. Last, there are severe limitations of the travelling speed.
It is an object of the invention to provide an improved sliding electrical contact device. It is a more particular object of the invention to provide such a device improved in respect of electrical losses, maximum accpetable linear speed and density of current.
According to an aspect of the invention, there is provided a device whose brush is formed by at least one bundle of stiff metal wires whose diameter is less than 80 microns, having a tight connection at one end and bearing on the track at the other end. It will be considered that a wire is "stiff" if, after it is given a straight form, it retains it and does not tend during handling to bend and to become tangled with other similar wires when it is placed in a bundle with them. The tight connection will generally be achieved by securing first end portions of the wires in a sole piece from which they project in a direction which is determined by the connection.
The results obtained with such a device are entirely different from those obtained in the case of wires of current diameter. The contact voltage difference is much lower (which reduces losses), the linear speed may be much higher, the friction is less, the admissible current density is much greater and the working life is lengthened. The difference may probably be explained as follows: the resilience of the individual wires is sufficient for them to remain in contact with the track even when passing over unavoidable irregularities. Since contact is retained continuously, there are no micro-arcing and micro-breakdowns and heating up on contact. The new and advantageous effect is lost if the diameter of the wire is increased beyond about 80 microns.
In practice, the wires will be made from copper alloy. Pairs of material which can be used for the brush and the track include cadmium bronze for the wires and cupronickel containing advantageously 10 to 20% by weight of nickel for the track. The appearance of oxide which rapidly affects the characteristics of the device is avoided by maintaining the brush and the track in a dry and non-oxidizing atmosphere, i.e. neutral, slightly reducing or rarefied. The latter condition is met during operation of the device in space or at high altitude. An argon or even nitrogen atmosphere may generally be suitable.
The connection between the wires and the sole piece can be described as of the "embedding" type; the free length of the wires should be short enough failing which they tend to lie down, which makes in particular reversal of running difficult. In practice, in particular when the device must be able to operate in both directions, a broad estimation is that the projection of the wires must not exceed 20 mm and, preferably, 15 mm; the embedding length may be greater than this value to ensure that they are securely held in place.
The size of the cross-section of the bundle or each bundle, the direction parallel to the travelling direction, should be limited to a value for which all wires are in contact with the track for fully benefiting of the advantages of the invention. Typically, the size of the bundle in the direction parallel to the travelling past will generally not exceed 10 mm.
The invention will be better understood from the following description of preferred embodiments thereof, given by way of non-limiting examples.